The invention relates to a powder mixture, containing formaldehyde- and/or formic acid-releasing compounds which have skin-tanning properties, which powder mixture contains an agent which forms sulphite ions.
It has been known for some time that compounds which have a ketol group 
preferably hydroxymethyl ketones, in particular dihydroxyacetone, have a self-tanning effect on human skin. This self-tanning effect is essentially based on a Maillard reaction between the ketol group of these compounds and the amino acids of the skin.
The hue obtained in this reaction can be further enhanced by adding certain adjuvants (for example EP 0 425 324).
Moreover, WO 91/12222 proposes to stabilize dihydroxyacetone (DHA) by coating at least part of it, in its dimeric form, with a water-impermeable composition. This publication furthermore proposes to employ formaldehyde-releasing compounds to prevent the bacterial degradation of DHA.
It has now been found that compounds, such as, for example, DHA, which have a ketol group, slowly release formaldehyde and/or formic acid in the cosmetic preparations. These are, in general, traces of formaldehyde or formic acid, i.e. formaldehyde contents of a few ppm, as a rule, 20-100 ppm.
It was therefore an object of the present invention to find pulverulent components which can be processed to give stable mixtures, in which the formation of formaldehyde and formic acid is completely suppressed.
Surprisingly, it has been found that, using agents which form sulphite ions, e.g., a source of sulfite ions, the formation of formaldehyde and formic acid can be completely suppressed in such pulverulent components and in formulations made with them.
The invention therefore relates to a powder mixture containing formaldehyde- and/or formic acid-releasing compounds which have skin-tanning properties, which powder mixture contains an agent which forms sulphite ions and, if appropriate, stabilizer.
The invention furthermore relates to the use of such a mixture in the manufacture of cosmetic or pharmaceutical preparations.
The invention furthermore also relates to the preparation of these mixtures, where at least one of the components, in particular the agent forming sulphite ions, is triturated or coated with the stabilizer.
The following are preferred embodiments:
a) mixtures in which the compound which has skin-tanning properties is dihydroxyacetone;
b) mixtures in which the agent which forms sulphite ions is a hydrogen sulphite, disulphite or dithionite, preferably an alkali metal hydrogen sulphite, alkaline earth metal hydrogen sulphite, alkali metal disulphite or alkali metal dithionite, in particular sodium hydrogen sulphite, sodium bisulphite or sodium dithionite;
c) mixtures which contain 45 to 99, preferably 80 to 90, % by weight of a formaldehyde- and/or formic acid-releasing compound (i.e., a compound with self-tanning properties), 0.1 to 50, preferably 1 to 20, % by weight of an agent which forms sulphite ions, and 0.1 to 5% by weight of a stabilizer, e.g., one which prevents the decomposition of the self-tanning compound, as described in WO 91/12222, in each case based on the total mixture;
d) mixtures in which the ratio by weight between the agent which forms sulphite ions and the formaldehyde- and/or formic acid-releasing compound is greater than 0.01;
e) mixtures which contain, as stabilizer, a stearate, in particular magnesium stearate;
f) mixtures which contain, as stabilizer, a film-forming agent;
g) mixtures which contain, as stabilizer, a cellulose derivative, in particular ethylcellulose or nitrocellulose;
h) mixtures which contain, as stabilizer, a higher alcohol, such as, for example, cetyl alcohol, a paraffin or another, conventional polymeric coating material.
Preferred compounds which have skin-tanning properties are those having a ketol group 
in particular dihydroxyacetone.
Preferred agents which form sulphite ions are metal sulphlites, metal disulphites or metal hydrogen sulphites and metal dithionites, such as, for example, sodium hydrogen sulphite, calcium hydrogen sulphite, sodium sulphite, potassium sulphite, sodium dithionite, zinc dithionite and sodium disulphite.
The mixture according to the invention is preferably employed in the manufacture of cosmetic preparations as a composition for tanning the human epidermis. Such a composition is in various forms which are conventionally used for this type. For example, it can be, in particular, in the form of oily or oily/alcoholic lotions, emulsions, such as a cream or a milk, in the form of oily/alcoholic, oily/aqueous or aqueous/alcoholic gels or in the form of solid sticks or else formulated as an aerosol.
It may contain cosmetic adjuvants which are conventionally used in this type of composition, such as, for example, thickeners, plasticizers, humectants, surfactants, fungicides, bactericides, preservatives, in particular alkyl 4-hydroxybenzoates, antifoams, perfumes, waxes, lanolin, propellants, colorants and/or pigments which impart color to the composition itself, UV filters for the protection against UV-A and/or UV-B rays and other ingredients customarily used in cosmetology.
It is highly preferred that the preparations according to the invention do not contain formaldehyde or formaldehyde-releasing compounds as bactericides.
An oil, wax or other fatty substance, a lower monoalcohol or a lower polyol or mixtures of these may be used as solubilizers. The particularly preferred monoalcohols or polyols include ethanol, i-propanol, propylene glycol, glycerol and sorbitol.
A preferred embodiment is an emulsion in the form of a cream or milk and includes, in addition to the skin-tanning compound, fatty alcohols, fatty acid esters, in particular fatty acid triglycerides, fatty acids, lanolin, natural or synthetic oils or waxes and emulsifiers in the presence of water.
Other preferred embodiments are oily lotions based on natural or synthetic oils and waxes, lanolin; fatty acid esters, in particular fatty acid triglycerides, or oily/alcoholic lotions based on a lower alcohol, such as ethanol, or a glycol, such as propylene glycol and/or a polyol, such as glycerol, and oils, waxes and fatty acid esters, such as fatty acid triglycerides.
The cosmetic composition can also be in the form of an alcoholic or aqueous gel which includes one or more lower alcohols or lower polyols, such as ethanol, propylene glycol or glycerol, and a thickener, such as silica.
The oily/alcoholic gels additionally contain a natural or synthetic oil or wax.
The solid sticks are composed of natural or synthetic waxes and oils, fatty alcohols, fatty acid esters, lanolin and other fatty substances.
Other suitable compositions are cosmetic sunscreen products which include at least one UV-B and/or UV-A filter.
Examples of suitable UV filters are cinnamic acid derivatives, benzylidenecamphor and its derivatives, p-aminobenzoic acid and its derivatives, salicylic acid derivatives, benzophenone derivatives and dibenzoylmethane derivatives. As a rule, the preparations contain 0.2 to 10% by weight of these UV filters.
If a preparation is formulated as an aerosol, the conventional propellants, such as alkanes, fluoroalkanes chlorofluoroalkanes are generally used.
If appropriate, the preparation may contain thickeners:
The thickeners or gelling agents known to the person skilled in the art can be used for thickening, examples being guar gum, heterobiopolysaccharides, xanthan gush scleroglucans, cellulose derivatives, such as, for example, methylcellulose, hydroxyethylcellulose, hydroxymethylpropylcellulose, alkali metal salts of carboxymetbylcellulose, and polyacrylic acids.
The mixtures according to the invention can be prepared big a range of processes known per se to the expert. This is because it has emerged, surprisingly, that all that is necessary for achieving a stabilizing effect is that contact between the individual components of the powder, i.e., the compound with self-tanning properties and/or the source of sulfite ions, is not too intimate.
Suitable stabilizers are therefore, for example, conventional lubricants, such as, for example, stearates, with which at least one of the powder components is triturated or ground. The at least partial coverage of the surface of the powder component, the self-tanning compound or the sulfite ion source, with the stabilizer achieved in this process suffices for the stabilization required. This trituration of the powder component with a lubricant can be carried out in conventional devices and mixers which are known to a person skilled in the art.
However, other agents which can be used for the stabilization are all conventional film-forming agents, such as, for example, cellulose derivatives, higher alcohols, paraffins and polymers. These agents are conventionally dissolved in a suitable solvent. Granules which can then be mixed with the second component are obtained by making the powder component into a paste together with the solution and subsequently drying and screening the product. However, it is also possible to spray, in a mixer, the powder component with the solution of the film-forming agent and drying the product, or other processes known to a person skilled in the art can be used.
It is preferred that at least one of the powder components is treated with the stabilizer. The question of which of the two components is to be treated can be decided by a person skilled in the art from a practical point of view, for example according to which component can be treated more easily in conventional mixers or granulating devices. Of course, it is also possible to treat both powder components.